Spot Welding
Questions and Answers
In resistance brazing we are not spot welding but we still are passing modest currents so an adequate force is necessary to avoid arcing. The main goal of the force is to hold the parts together for current flow and avoid arcing so the braze alloy is not arced out of the joint. This is usually a modest force developed by trial with set up parts. Joule Heating H= I2rt will then generate the energy necessary for the braze material to melt. When molten braze metal is visible in the filet the current can be turned off and allowed to cool under pressure until solidified. Then the electrode can open and the part is ready to be ejected or removed. The temperature that the part reached at the joint is lower than it would have been if spot welded but it will be hot so caution is required.
Resistance brazing is not spot welding so the electrode face geometry does not have to concentrate the heat into a small area. Frequently the electrode face is machined to conform to the surface that it is in contact with. Frequently the electrode will contact the entire work piece surface. In some cases the electrode faces is milled out so the work piece can be nested into the face of the electrode in order to insure proper part placement and alignment. A small vacuum is sometimes pulled through the center of the electrode to hold the part in place before the electrode closes. This is very common for small part brazing.
Resistance brazing is a variation of the resistance welding process. Joules Law H= I2 RT provides the heat just as in spot welding. There is a squeeze weld and hold cycle. There are electrodes. To differentiate the process a thin piece of braze allow is placed between the parts being joined. The goal is to heat the braze material until it melts and bonds to both mating substrates. Then the current is turned off and the braze material solidifies. The result is a brazed joint bonding the parts together. Low force holders are frequently used.
Flash and butt welding both use the work piece as the electrode. They both use a clamp to hold the parts and apply force. This clamp carries current so it must conduct current and dissipate heat. This makes copper alloys a first choice. In many cases the Group A copper alloys will be used for this application. If mechanical wear is an issue Class 3 might replace Class 2.
Spot welding electrodes have a large influence on the resistance welding process. Their alloy and face size contribute to the development of the proper weld nugget. All of this assumes that the electrodes have been installed properly. This means proper water cooling and water tube location and good alignment. Alignment means that the face of the electrode is square and parallel to the part being welded. In addition the upper and lower electrodes faces should line up vertically with each other. The goal is to apply all forces on the same central axis. This prevents the electrodes from imparting and twist or distortion into the part during the welding operation.
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